In message <[email protected]>,
dated Thu, 10 Sep 2015, Ken Wyatt <[email protected]> writes:
Typically, the USB connector shield is soldered directly to the digital
return plane. Is isolating the connector ground from the digital return
plane and inserting a ferrite bead between the two the answer?
I don't think so, because the CM is also on the wires and they couple
the CM back to the shield downstream of the ferrite bead.
While it may be very difficult to do on a large board, the best place to
connect the shell to, via a minimal lead inductance, is upstream of
every source of CM emission, which, especially if he product has no
safety earth, is some point on the ground tracks and plane, whose
capacitance to local ground is highest. However, what 'local ground' do
you measure the CM voltages with respect to? It seems to depend on the
individual PCB design.
In practice, you move the shield ground connection around the board and
any metal work until the CM emission is as small as possible. But a
different point may be best at different frequencies.
If it was too easy, it wouldn't be so fascinating. (;-)
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
When I turn my back on the sun, it's to look for a rainbow
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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